For almost half a century, scientists have struggled with plutonium contamination spreading further in groundwater than expected, increasing the risk of sickness in humans and animals.

It was known that nanometer-size clusters of plutonium oxide were the culprit, but no one had been able to study its structure, nor find a way to separate it from the groundwater.

Scientists at the U.S. Department of Energy’s Argonne National Laboratory, in collaboration with researchers from the University of Notre Dame, used high-energy x-ray beams from the X-ray Operations and Research/BESSRC 11-ID-B beamline at the Argonne Advanced Photon Source to finally discover and study the structure of plutonium nanoclusters. Their research results were published in Angewandte Chemie International Edition.

“When plutonium forms into the clusters, its chemistry is completely different and no one has really been able to assess the clusters' composition, how to model them, or how to isolate them,” said Argonne senior chemist Lynda Soderholm. “People have known about and tried to understand the nanoclusters, but it was modern analytical techniques and the APS that allowed us understand what it is.”

The nanoclusters are made up of exactly 38 plutonium atoms and have almost no charge. Unlike stray plutonium ions, which carry a positive charge, they are not attracted to the electrons in plant life, minerals, etc., which stopped the ions’ progression in ground water.

Our current understanding has been based on the free-plutonium ion, creating discrepancies between what is expected and what is reality. Soderholm said that with knowledge of the structure, scientists can now create better models to account for not only free-roaming plutonium ions, but also the nanoclusters.

The clusters also are a problem for plutonium remediation. The free ions are relatively easy to separate out from groundwater, but the clusters are difficult to remove.

“As we learn more, we will be able to model the nanoclusters and figure out how to break them apart,” Soderholm said. “Once they are formed, they are very hard to get rid of.”

Soderholm said other experiments have shown some clusters with different numbers of plutonium atoms and she—together with her collaborators S. Skanthakumar, Richard Wilson, and Peter Burns of Argonne’s Chemical Sciences and Engineering Division—plans to examine the unique electric and magnetic properties of the clusters. – Brock Cooper

This research is supported at Argonne National Laboratory by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division, and by the Material Sciences Division for the Advanced Photon Source studies, all under contract DE-AC02-06CH11357. Research at the University of Notre Dame was supported by the National Science Foundation, Environmental Molecular Science Institute (EAR02-21966).

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